This sentence tells nothing about the future. “Will” here is used to describe what happened in the past / present.
No chance of finding him sober now; he will have been drinking all day.
Can you explain me this kind of usage of will?
“Will” has several functions. One of them is to express likelihood. As the Cobuild English Grammar states:
You use ‘will’ when you are assuming that something is the case and you
do not think there is any reason to doubt it.
You can use will + continuous in this way to express your assumption about the future, present and past.
No doubt he’ll be drinking when we arrive.
Knowing him he will be drinking already.
Small wonder he had a hangover. He will have been drinking all day the
Source : Link , Question Author : Graduate , Answer Author : Shoe